The Applicant has described an underwater positioning system in co-pending Australian Patent Application number claiming priority from Australian Provisional Application No. PR3894, the contents of which are herein disclosed by way of reference. In an underwater environment, such as that disclosed in the Applicant's co-pending application, it is necessary for signals transmitted from an underwater station to be received with sufficient signal power.
One of the main factors reducing the effectiveness of underwater communications is that of ray bending. Ray bending occurs in a long-range deep water tracking system, (i.e. Horizontal propagation of acoustic signals). Non-uniform water density causes horizontal acoustic signals to refract vertically to the surface or to the ocean bottom. This effect limits the maximum horizontal range of deep-water tracking. If a beacon is deployed on the bottom of the ocean, ray bending will limit the maximum range, as the acoustic signal will refract to the bottom of the ocean well before the acoustic signal reaches the surface. The traditional solution to this problem is to deploy a series of beacons closer together, which has obvious increased cost and maintenance implications.
An alternative solution to the ray-bending problem is to transmit very high power beacon signals. When the acoustic signal refracts into the ocean floor a small proportion of this signal will be reflected up towards the surface of the ocean. If the acoustic signal is loud enough the signal will be detectable at the surface. This type of tracking is unreliable and it is difficult to predict the power of the acoustic signal at the receiver. The other problem with high power signals is that a larger energy source is required at the beacon. Either a large and expensive energy source is used or the operational life of the beacon is reduced, as approximately a ten times increase in power is required.